Whenever I have to RSVP my regrets over the phone, I sound like an idiot. When I call to let the host know I will be unable to attend, I always end up practically giving my life story which then comes across as sounding like an excuse or I would rather be pulling my teeth out than going to the party. What do you say when declining an invitation that is still gracious but direct?

"Thank you so much for the invitation, it sounds like it is going to be such a great event! I would love to go, however I am not going to be able to attend due to prior commitments. (Don't take a breath here, or people ask questions, just continue) Please tell me when I can see you next, I miss seeing you...we really should do lunch...speaking of lunches, have you heard of that new resturant? They have the best Salmon in the world! I actually just found a recipie for samon....you have to try it, do you like salmon?..."

Basically get your point across then you can't go....then babble on and change the topic...you were precise, concise and still seem friendly. I am an expect on this, especially since I have an anxeity disorder.
DO NOT USE THE WORDS 'BUT' or 'UNFORTUNATELY'
They have a negative connotation.